Lens



Nov. 29, 1960 E. K. CLARK ETAL 2,961,925

LENS

Filed Oct. 2l, 1943 ATTORNEY 22 '/2 Dey rees LENS Earl K. Clark and Harold P. Allen, Mansfield, Ohio, as-

signors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed oet. 21, 194s, ser. No. 507,122

14 claims. (c1. sat-57) Our invention relates to improvements in apparatus for bombs and projectiles, and more particularly to lenses for proximity fuses of bombs and projectiles.

The effectiveness of anti-aircraft fire is very much increased if the explosive projectile, bomb, or other pro pelled explosive charge is caused to explode, if a direct hit cannot be scored which is usually the case, in the proximity of the target that is to be destroyed.

Rocket propelled projectiles or bombs cannot be accurately controlled as to direction of ight. It is, therefore, highly desirable to cause such projectile to explode when anywhere near the target. f

The best manner of doing this is by means of photoelectric cells, commonly known as electric eyes. It is vhighly desirable to make the seeing ability of such device most effective.

One object of our invention is the provision of optical means for improving the operation of photoelectn'cally actuated proximity fuses.

Another object of our invention is the provision of a leus particularly adapted for a photoelectric cell for causing the operation of a detonator.

A further and broad object of our invention is the provision of a lens for gathering light from a particular region of space with reference to the direction of the longitudinal axis of the bomb, which, in the absence of tumbling, is the direction of ight of a bomb or projectile.

Another object of our invention is the provision of an annular lens to gather light in a cone, or funnel-shaped portion of space, which has its vertex at the location ot a light-sensitive device as a photoelectric cell.

A still further object of our invention is the provision of an annular lens of transparent polymers, interpolymers and copolymers o f derivatives of acrylic and methacrylic acids, derivatives of alkyl alcohol, polystyren, Lucite, or similar resins.

Still other objects and advantages of our invention will become more apparent from a study of the following specification and the accompanying drawing, in which:

Figure l is a plan view of my novel annular lens;

Fig. 2 is a sectional view, taken on line II-II ot Fig. 1, and looking in the direction of the arrows at the ends of the section line; and

Fig. 3 shows a curve illustrating the desirable qualities of my lens.

In Fig. .1, the annular lens 1 is shown in plan view and shows screw-threaded openings 2, 3, 4 and 5 for receiving the opaque metallic nosepiece 6 and other apparatus of the projectile not forming part of this invention.

Certain resins, as Lucite, methacrylate, and others above-mentioned, have excellent transparency, good indices of refraction, and above all are not easily shattered as glass would be. Furthermore, resins can be molded or formed to desired shapes much more readily than other known transparent materials suitable for lenses.

We prefer to mold our lenses substantially to the right shape and dimensions by either compression molding or injection molding.

nited States Patent O cc 2,96L925 Ice Patented Nov. 29, 1960 'Ihe annular lens is fit snugly between the nosepiece 6 and the body 7 of the projectile. The diameter of the lens is so chosen that the crown perimeter formed by the locus of the points as point C at the highest portion of the lens curvature lies below the outside ogive of the projectile as the loci of points O. In other words, the circle formed by all points corresponding to point C lies inside, but slightly below, the circle formed by all points corresponding to point O. This lens thus provides the advantage in its final assembly of minimum exposure to damage. Only the small portion disposed between regions x and y is exposed in the outside ogive of the projectile. The metal body 7 and the metal nosepiece or cap 6 are so positioned to define the small width between regions x and y. The lens thus aiords greater strength inasmuch as it occupies a position intermediate the ends of the unit, or projectile, and is low in co'st as well as economic in its use of very strategic material that is now vitally needed in war production.

For defense weapons that destroy themselves as do bombs, it is extremely important that the device function exactly when it is supposed to function and yet also be constructed of material of the lowest possible cost. We have found that some of the resins herein mentioned have the kind of index of refraction to make them very suitable for lenses to be used in projectiles, and at the same time combine ruggedness and low cost. We have found that the most effective manner of making our lens is by screwing the blank, which already has substantially the shape the finished lens will have, on a threaded mandrel in a simple turning lathe. The outside convex surface defined between the regions x and y in Fig. 2 is then formed by a tool which possesses the proper optical curvature for the particular refracting surface, and the inside bore at region 8 is machined out in a circular hole leaving a small shoulder at 9, at which region the inner diameter is somewhat less than the diameter of the opening machined out at region 8. When the lens has thus been provided with the outside refracting surface and the necessary inside machining, special paint indicated by 10, possessing properties of good adhesion, great opacity or high pigment density and chemical compatibility with the methacrylate or other resin, is sprayed over all inside surfaces excepting the threads. The lens is then repositioned on the lathe and the shoulder 9 is machined on the surface parallel to the longitudinal axis of the lens to remove all the paint at region 11, and this surface is then, after careful machining, given a high polish with n precise honing tool.

The lens thus in ight, assuming the direction of flight indicated by the arrow 12, gathers light in a funnelshaped region which is always, by design, 221/2 above a plane normal to the longitudinal axis of the lens; that is, line 13 is 221/v above line 14. The light thus gathered is focused on region 11, and that region 11 appears as a very-bright band of light illuminating the region within the lens and particularly point P, the position of the photocell for actuating the detonator.

By means of the few simple operations in the steps of manufacture, we provide a lens of high light transmission v and good resolving properties. The curve shown in Fig. 3 gives the distribution of light from a point source as received on a cathode placed at the proper position as point P, and shows the relationship of the amount of light in percent of maximum with variation in the angle designated as 2.21/2". To the right of the region marked 22% we may assume is the region between lines 13 and 15 of 22% and to the left we may consider the region between lines 13 and 14. It will thus be seen that as the object or a source of light moves in the region defined by the angles between lines 14 and 15, the light transmitted to the photocell varies as indicated by the curve. The

significance of this curve in terms of operation may be defined on the basis of angular width at v80, 50 and 5% maximum light transmission. The point of maximum transmission as M we may consider as 100% transmission of light. The resolving power of this type of lens is the greatest when this curve becomes truly rectangular, as would be indicated by the vertical lines drawn through points Q and R. A lens with perfect resolving powers would be one defining a curve having vertical sides drawn through the points defining angular width at 50% light transmission. It has been found, and specifications for defense use usually specify, that for a given angular width 941/% the light transmission must be 50%; for a,given angular'l width of 21/2 the light transmission, must be at least 80%. Further, for an angular width of 12 the light transmission must be less than 5%. It will be noted that the resolving power of our lens is far superior to these requirements, givingaverage values as follows: At 80% transmission, the. angular width is from 3% to 31A which is considerably greater than the 2% minimum requirement; at 50% light transmission, which is the basis for comparison, the angular width is the same as the requirements, whereas at 5% light transmission, the angular width is from 7 to 9. It will thus be apparent that a lens made in accordance with our construction and selection of the material is thus very much superior in its resolving power. Any lens that in the process of manufacture or the selection of material made, or both, at the angular width specified comes too close to the specification to the required angularwidths, naturally requires inspection of every lens to determine whether or not it can be used in a weapon, and the number thus rejected may be as high as 50%. Our lens provides such a wide margin in the direction of a lens having perfect resolving powers from the minimum requirements that once the cutting tool has been properly designed and the operations properly carried out no inspection of the lens is necessary. One of the chief reasons for the resolving power of our lens is the narrowing down of the tails of the angular distribution curve with its attendant minimizing or elimination of sun difficulties in operation. A lens thus constructed in accordance with our procedure and of the materials selected provides improved optical performance by simple and inexpensive means. Further, a greater accuracy is obtained on the look-forward angle of 22% the slit width which defines the angle at 50% light transmission, with the result that the photocell which is positioned at P or this general region will cause operation of the detonator at exactly the instant when the target is in the correct position with reference to the moving bomb or projectile.

The curvature of the region defined between x and y will depend, of course, on the refracted index of the material. We have found that for Lucite an excellent cutting tool for making the light receiving surface can be made by drilling a hole through the tool material at an angle of 38 with reference to a plane normal to the longitudinal axis of the tool material and normal to the top surface of tool material. The drill will thus form a hole in the cutting tool material elliptical in shape, the eccentricity of the ellipse being, of course, a function of the refracted index of the material used.

A particular optical material actually used in practice is the well-known Lucite, but we do not wish to b e limited to this particular resin nor to the exact details of the shape of the curve of the light receiving surface of the lens nor other details, but wish to be limited only by the .scope of the claims hereto appended.

We claim as our invention:

\1. In an optical device, a lens in the shape of a ring having a cylindrical inner surface and a curved outer light gathering surface, the curvature of a given portion of the outer surface in every plane including the ring axis being in the shape of a portion of an ellipse of fixed dimensions and disposition with reference to the ring axis, said given portion of the outer surface being the sole light gathering surface and being symmetrical with reference to the major axis of said ellipse to thus focus all the light gathered by the light gathering surface on the major axis of the ellipse, said ring being so dimensioned that lthe cylindrical inner surface falls at the focal points of the major axes to thus form a narrow band of a relatively highly illuminated region in the inner cylindrical surface of the lens.

2. In an optical device, a lens in the shape of a ring having a cylindrical inner surface and a curved outer light gathering surface, the curvature of a given portion of the outer surface in every plane including the ring axis being in the shape of a portion of an ellipse of fixed dimensions and disposition with reference to the ring axis, said given portion of the outer surface being the sole light gathering surface and being symmetrical with reference to the major axis of said ellipse to thus focus all the light gathered by the light gathering surface on the major axis of the ellipse, said ring being so dimensioned that the cylindrical inner surface falls at the focal points of the major axes to thus form a narrow band of a relatively highly illuminated region in the inner cylindrical surface of the lens, and an opaque adhesive material disposed on the lens except at the light gathering surface and at the narrow band of highly illuminated surface.

3. A lens for a projectile, said lens being in the shape of a`ring having an annular inner light emitting surface and a concentric light gathering surface of a given width at the outer region of the ring having a fixed elliptical contour in every plane including the ring axis to thus focus the light gathered at a given point on the major axis whereby a narrow ring of illumination is produced, the thickness of the lens rin-g being so chosen with respect to the index of refraction thereof that the narrow ring of illumination falls in the inner surface of the ring whereby the region within the ring is relatively highly illuminated.

4. A lens for a projectile, said lens being in the shape of a ring, a light gathering surface of a given width at the outer region of the ring having a fixed elliptical contour in every plane including the ring axis, said lens having an annular inner surface concentric with said light gathering surface so disposed with reference to the refractive power of said lens as to receive the light gathered at a given point on the major axis whereby a narrow nng of illumination is produced, the thickness of the lens ring being so chosen that the narrow ring of illumination falls in the inner surface of the ring whereby the region within the ring is relatively highly illuminated, and an opaque medium on the lens except at the light gathering surface and at the focal region.

5. In an optical device, a lens in the shape of a ring having an inner surface of revolution generated by the revolution about the ring axis of a portion of a plane curve disposed in a plane including the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including'the ring axis, the said portion of said ellipse being symmetrical with reference to an axis of the ellipse to direct the optical axis of said lens through said inner surface, the lens being so proportioned with reference to the index of refraction of the lens material that the light gathered by the outer surface is focused substantially on said inner surface.

6. In an optical device, a lens in the shape of a ring having an inner surface of revolution generated bythe revolution about the ring axis of a portion of a plane curve disposed in a plane including the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the major of the ellipse to direct the optical axis of said lens through said inner surface, the lens being so proportioned with reference to the index of refraction of the lens material that the light gathered by the outer surface is focused substantially on said inner surface, and an opaque adhering material on the lens except at the light gathering surface and at a narrow circumferential region on the inner surface.

7. In an optical device, a lens in the shape of a ring having an inner surface of revolution generated by the revolution about the ring axis of a portion of a plane curve disposed in a plane including the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, thel said portion of said ellipse being symmetrical with reference to an axis of the ellipse andvdisposed to direct the optical axis through said inner surface, the dimensions of the ring being so chosen with reference to the index of refraction of the particular lens material selected that the light gathered by the light gathering surface is focused in the inner surface of revolution.

8. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape lof a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the maior axis of the ellipse, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface is focused on the narrow inner cylindrical surface of the focal region.

9. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape of a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the major axis of the ellipse, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface is focused on the narrow inner cylindrical surface of the focal region, the major axis of the surface generating ellipse being disposed at a fixed angle with reference to a plane normal to the ring axis.

10. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape of a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the major axis of the ellipse, the major axis of the surface generating ellipse also being disposed at a xed angle of 221A with reference to a plane normal to the ring axis, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface is focused on the narrow inner cylindrical surface of the focal region, and that more than 80% of the light coming toward the light gathering surface from a` region at an angle of 256 from the major axis is transmitted to the focal region, that 50% of the light coming toward the light gathering surface from a region at an angle of 4% from the maior axis is transmitted to the focal region, and that less than 5% of the light coming toward the light gathering surface from a region at an angle of 12 from the major axis is transmitted to the focal region, all considered on the basis of 100% light transmission to the focal p oint for light coming toward the light gathering surface from the region in line with the major axis.

ll. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape of a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the major axis of the ellipse, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface isfocused on the narrow inner cylindrical surface of the focal region, and an opaque adhering material covering the lens except at the light gathering surface and the focal region.

12. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape of a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to the major axis of the ellipse, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface is focused on the narrow inner cylindrical surface of the focal region, vthe major axis of the surface generating ellipse being disposed at a xed angle with reference to a plane normal to the ring axis, and an opaque adhering material covering the lens except at the light gathering surface and the focal region.

13. In an optical device, a lens of resinous material having a relatively large index of refraction and formed in the shape of a ring, said lens having a highly polished quite narrow inner circular cylindrical surface concentric with the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the 'said portion of said ellipse being symmetrical with reference to the major axis of the ellipse, the major axis of the surface generating ellipse also being disposed at a fixed angle of 221/2 with reference to a plane normal to the ring axis, the dimensions of the surface generating ellipse and the disposition of the narrow circular cylindrical surface being so chosen with reference to the index of refraction of the resinous material selected that the light gathered by the light gathering surface is focused on the narrow inner cylindrical surface of the focal region, and that more than of the light coming toward the light gathering surface from a region at an angle of 21/z from the major axis is transmitted to the focal region, that 50% of the light coming toward the light gathering surface from a region at an angle of 41/2 from the major axis is transmitted to the focal region, and that less than 5% of the light coming toward the light coming toward the light gathering surface from a region at an angle of 12 from the major axis is .transmitted to the focal region, all considered on the basis of light transmission to the focal point for light coming toward the light gathering surface from the region in line with the major axis and an opaque adhering material covering the lens except at the light gathering surface and the focal region.

14. In an optical device, a lens in the shape of a ring having an inner surface of revolution generated by the revolution about the ring axis of a portion of a plane curve disposed in a plane including the ring axis, and having an outer light gathering surface generated by the revolution about the ring axis of a portion of an ellipse disposed in a plane including the ring axis, the said portion of said ellipse being symmetrical with reference to an axis of the ellipse, the dimensions of the ellipse for forming the light gathering surface and the radial dimensions of the ring with reference-to the index of refraction of the resinous lens material selected being such that the light gathered is focused in the inner surface of revolution.

Reference: Cited in the tile of this patent UNITED STATES PATENTS 

